2009MNRAS.397L..46H

M4 and NGC 6397 are two very similar galactic globular clusters, which differ mainly in their surface brightness profile. M4 has a classic King-like profile, whereas NGC 6397 has a more concentrated profile, which is often interpreted as that of a post-core collapse cluster. In previous papers, however, the authors found that M4 is also a post-core collapse cluster, and concluded that the main reason for the difference between the two surface brightness profiles is fluctuations. This conclusion was reached on the basis of Monte Carlo models, however, and in the present Letter we verify that similar fluctuations occur in N-body models. The models were initialized by generating initial conditions from the Monte Carlo model of NGC 6397 at the simulated age of 12 Gyr, and one was followed for 1 Gyr. The new models help us to clarify the nature of the fluctuations, which have the nature of semiregular oscillations with a time-scale of the order of 10⁸years. They are influenced by the dynamical role which is played by primordial binaries in the evolution of the core.